The invention relates to a double screw for a four-part activator intended for simultaneous independent activation of the upper and lower jaw, having a first expansion screw to be associated with the frontal and lateral dentition of the upper jaw and a second expansion screw parallel thereto, to be associated with the incisor and lateral dentition of the lower jaw, as well as a web connecting the two screws vertically to their axes.
Such a double screw is described in Professor Dr. Felix Ascher's book Praktische Kieferothopadie, Urban & Schwarzenberg, Munich-Berlin-Vienna 1968, p. 110 and 111. The screw is used when the differential compression in the upper and lower jaw is so high that the necessary expansion in the upper and lower lateral dentition can hardly be achieved by ordinary means. The double screw comprises two screws connected by a sturdy vertical web. One screw is located in the frontal division of the upper jaw, the other in the incisor division of the lower jaw. The connecting web is oriented with the center of the bite. The assembly is imbedded in the wax form of the activator. The single screws used are those employed in active expansion plates. The plate is parted at the midline and parallel to the occlusal plane, the biting groove for the lower incisors being retained in the lower jaw part of the plate structure. Four parts result, held together by the aforesaid vertical web.
The known double screw has the disadvantage that firstly the activator parts, for secure imbedding of the double screw, must be of massive construction, restricting the clearance for the tongue, and that secondly, owing to the preassigned size of the vertical web, in smaller jaws the double screw must be arranged farther away from the front teeth, further confining the lingual bed.